A voltage reducing power supply circuit (series regulator) detects its output voltage, and compares the detected voltage with a predetermined reference voltage. In response to a voltage comparison result, the voltage reducing power supply circuit controls a conduction condition under ON state of a control transistor, namely controls a voltage between the collector and the emitter of the control transistor.
Specifically, when the detected voltage is higher than the reference voltage, the voltage reducing power supply circuit increases the collector-emitter voltage. Conversely, when the detected voltage is lower than the reference voltage, the voltage reducing power supply circuit decreases the collector-emitter voltage. As a result, while the output voltage can be stabilized, the voltage reducing power supply circuit can produce the target output voltage from the input voltage in high precision.
However, in the voltage reducing power supply circuit, the input voltage is reduced to produce the output voltage. Therefore, a relatively large power loss may necessarily occur. The larger a difference between the input voltage and the output voltage becomes, the larger the power loss is increased.
On the other hand, a switching power supply circuit (switching regulator) is also known as a power supply circuit capable of reducing a power loss. In this switching regulator circuit, an input voltage is interrupted by turning on and off a switching device. Then, the interrupted input voltage is smoothed by using a coil and a capacitor. Since the switching power supply circuit adjusts the output voltage by turning on and off the switching device, the power loss can be reduced.
However, in the switching power supply circuit, noise such as ripple voltages may be produced in connection with on and off operations of the switching device.
As a consequence, as disclosed in, for instance, JP-A-6-335238, a switching power supply circuit is connected in series with a voltage reducing power supply circuit. In accordance with this conventional electric power supply apparatus, after the input voltage has been previously reduced by the switching power supply circuit, the reduced input voltage is applied to the voltage reducing power supply circuit. As a result, lowering of the power loss can be improved. Further, since the noise caused by the switching power supply circuit is smoothed by the voltage reducing power supply circuit, the suppression of the noise can be improved.
When an input voltage applied to the switching power supply circuit rises, the switching power supply circuit may rapidly raise an output voltage thereof. In this case, a very large current may flow into a smoothing circuit constructed with a coil and a capacitor due to the raised output voltage, as compared with the current of the steady state. In particular, the coil provided in the smoothing circuit is stressed by the large current. As a result, reliability may be lowered.
To avoid this stress, a soft starting circuit or the like is provided in a switching power supply circuit so that, when an input voltage is applied to the switching power supply circuit, an output voltage thereof may gradually rise. This soft starting circuit is effectively operated in, for example, a circuit for producing a pulse signal which drives a switching device to generate such a pulse signal, the pulse width of which is gradually prolonged.
As described above, when the soft starting circuit is provided in the switching power supply circuit, the improvement in the reliability may be conspicuously achieved. However, a delay time will occur until the output voltage rises and then becomes stable. As a consequence, in the case that the switching power supply circuit is connected in series with the voltage reducing power supply circuit, a delay time may be produced when the output voltage of the voltage reducing power supply circuit rises, which is mainly caused by, especially, a rising delay of the output voltage in the switching power supply circuit. As a result, when a load circuit connected to the conventional power supply apparatus is initiated, a very long time duration may be required until the load circuit is activated, depending upon a magnitude of a required load activating current.